The main objective of these studies is to delineate antigens of the rheumatoid arthritic (RA)-derived synovial fibroblasts using immunochemical means and heterologous antisera to the RA cells maintained in culture. The major antigen of these cells shown to be identical to the transformation-sensitive cell surface glycoprotein, fibronectin, which is immunochemically similar to plasma cold-insoluble globulin (CIG). A different molecular form of CIG was found by immunoelectrophoretic analysis in synovial fluids and shown to be associated with fluids of RA origin. A large number of heterologous antisera were produced to the RA synovial cells and absorbed with known cross-reactive antigen sources. Certain of the antisera showed specificities to: a synovial fibroblast antigen; an antigen found in greater quantities on non-RA synovial cells; and a possible RA synovial fibroblast-associated antigen. These antigens are being isolated by immunoprecipitation for the purpose of eliciting further more specific antisera. Other studies include: CIG is found in cryoglobulins and its association with immunoglobulins and complement components is being studied. The specificities of RA synovial tissue-derived immunoglobulins are being determined to a number of antigen sources. In addition, antisera produced to a platelet-derived connective tissue activating peptide and mitogen are being used to assay quantities of the mitogen in various biologic sources.